1. Field of the Invention
The invention relates to an intake air device provided for an internal combustion engine.
2. Description of the Related Art
In internal combustion engines, an intake air device is provided to supply air-fuel mixture to a combustion chamber. This intake air device includes intake passages that connect a surge tank to intake ports in a cylinder-head. The intake passages are provided for respective intake ports (i.e., respective cylinders).
Japanese Patent Application Publication No. JP-A-2001-82155 describes an intake air device where the length of each intake passage varies among predetermined values. Japanese Patent Application Publication No. JP-A-2001-509562 describes an intake air device where the length of each intake passage can be continuously changed.
In internal combustion engines, when the intake valve of each cylinder is opened to introduce air-fuel mixture into the combustion chamber during the intake stroke, pulsating intake air passes through the intake passage. Because the intake air is pulsating in the intake passage, high-density air and low-density air are alternately present in the flow of the intake air. The cycle of the pulsation of the intake air varies depending on the shape, length, cross sectional area, and the like of the intake passage. Meanwhile, the time point where the intake valve is opened and the time point where the intake valve is closed vary depending on the engine speed.
If high-density air reaches the combustion chamber in the last stage of the intake stroke, the intake air continues to flow into the combustion chamber due to inertia even after a piston passes a bottom dead center. This effect is referred to as “inertia supercharging effect”. This improves the efficiency in filling the combustion chamber with intake air, and the output of the internal combustion engine.
In each of the aforementioned intake air devices where the length of each intake passage is variable, the length of each intake passage is changed according to the parameters indicating the operating state, such as the load and speed of the internal combustion engine. As a result, the aforementioned inertia supercharging effect can be obtained at all engine speeds. This improves the efficiency in filling the combustion chamber with intake air.
In each of the intake air devices, the maximum length and cross sectional area of each intake passage are fixed to respective predetermined values, and only the length of each intake passage can be changed. Therefore, the size of the intake air device needs to be increased to improve the efficiency in the filling the combustion chamber with intake air at all engine speeds, for the reason described below.
If the cross sectional area of each intake passage is reduced to optimize the amount of intake air supplied at low engine speed, an adequate amount of intake air cannot be supplied at high engine speeds. If the cross sectional area of each intake passage is increased to optimize the amount of air supplied at high engine speed, the maximum length of each intake passage needs to be maximized so that the appropriate amount of intake air is supplied at low engine speeds. Accordingly, the size of the component that constitutes each intake passage needs to be increased. Therefore, if the space in a vehicle is insufficient, the intake air device cannot be installed.
Japanese Patent Application Publication No. JP-A-3395009 describes an intake air device that includes two intake passages for each cylinder. In the intake air device, one of the intake passages is used at low engine speed, and the other intake passage is used at high engine speed.
This intake air device needs to be designed such that the length and cross sectional area of each of the two intake passages become appropriate at all engine speeds. This increases the size of the intake air device. As a result, if there is insufficient space in a vehicle, the intake air device cannot be installed.